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Life on Mars

We have wondered for centuries whether our neighbouring planet is home to life, just like our own. In the 19th century, the idea of men on Mars took off after astronomer Giovanni Schiaparelli observed straight lines on Mars, which he called canali. This means channels, but it was wrongly translated into English as canals, and many people interpreted the discovery as evidence of intelligent engineering work. The lines were later revealed to be illusory, perhaps caused by streaks of dust carried by the wind.

For life to exist on Mars, there must be liquid water. There is plenty of water on Mars, but most of it is frozen in the polar ice caps and buried underground. Despite the chilly temperatures, there could be liquid water underneath the surface ice in some places, since salt lowers the freezing point of water. Satellite data suggest there is a permanent lake, 20 kilometres across, hidden beneath the south pole.

Dark streaks called recurrent slope lineae form on Mars’s slopes during warm seasons, and these have been interpreted as flowing briny water. But liquid water on the surface would be hard to explain – and the flows may just be tumbling sand.

If there were once oceans on Mars, they would not have stuck around for long, leaving little time for life to evolve. A barrage of asteroids may have brought the water there, only for it to freeze and disappear within a few hundred million years.

Aside from water, another main clue to look for is the presence of organic compounds, which form the basis for all life on Earth. NASA’s Curiosity rover has found complex organic molecules, which could have been made by ancient life forms.

Methane gas is a possible sign of life: on Earth, most of it is produced by microbes, although it can also be produced by geological sources. We have spotted tantalising glimpses of methane on Mars a few times over the years, but it has been difficult to confirm the detections with other instruments.

In 2019, Curiosity detected the largest amount of methane ever found on Mars. Two satellites were observing the area simultaneously, so it may be possible to confirm the discovery with independent measurements.

Under planetary protection rules, all Mars-bound spacecraft must undergo a rigorous sterilisation procedure to make sure we don’t contaminate Mars with microbes from Earth. These protocols add significantly to the cost of space missions, and some researchers argue that the rules should be relaxed. Sam Wong